Conventionally, oils or lards are mixed and contained in water discharged from a general house, a commercial cooking place, or a building, or water discharged from a conduit for a sewage or waste liquid treatment facility of a public establishment. Discharged water which is such a liquid mixture in which water and oil are mixed with each other is the cause of clogging of a sewerage pipe by fixation of the oil, or the cause of an odor occurring by oxidation of the oil. In addition, there are also problems in that the function of the public sewerage facility is hindered or an oil clot (white solid matter) from a sewerage facility is spilled to a harbor after heavy rain, for example. Thus, in each district, measures in which restaurant companies have to install a gathering machine which separates and collects oils, fats, and the like in a liquid mixture, and thus oils or lards are not spilled to the sewerage are also taken.
Treatment of separating an oil-water mixed liquid into oil and water is performed as waste liquid treatment in, for example, food manufacturing, fiber treatment, mechanical processing, and petroleum refining, and as an oil collection work performed in a case where an oil is spilled into a river, the sea, and the like due to, for example, an accident.
In addition, for example, when crude oil is drilled for, a method in which the seawater is injected to an oil layer of a stratum, and pressure of non-aqueous oil is increased, and thus an output is ensured is generally performed. “Water accompanying drilling in the oil field” which is water used in such drilling for crude oil contains a large amount of non-aqueous oil. Thus, treatment of removing the non-aqueous oil is performed, and then the non-aqueous oil is scrapped. However, because the non-aqueous oil is the cause of contaminating the ocean, lakes and marshes, and the like, recently, restrictions over the content of the non-aqueous oil in discharged water is reinforced. In a country or a district in which the restrictions are strongest, the content of the non-aqueous oil is required to be less than 5 mg/L.
As the conventional oil-water separation method, for example, the following methods are known: separation by using a flocculant; adhering and separation; centrifugation; pressure floatation separation; an electrolysis floatation method; coarse granulation and separation by using a coalescer (for example, see Patent Document 1); and separation by microbial degradation.
In a case of a separation method using a flocculant, there is a problem in that expenses are continuously required, and treatment of filtered agglutinates also takes much labor and cost. A case by a machine such as a centrifuge and a case by pressure floatation separation are effective for treating a large amount or for large-size utilities. However, the above cases have a problem in that it is difficult to perform in a limited space. In the electrolysis floatation method, there is a problem in that complex control, for example, changing an applied power in accordance with electrical conductivity and the treated amount of a treatment liquid is required for stably performing oil-water separation. In the coalescer method, a filter having a network structure of ultrafine fiber is used. Thus, there is a problem in that clogging normally occurs in maintenance management. In a separation method using a microorganism, there is a problem in that it takes time, and maintenance is intricate.
Water treatment by using a separation membrane which uses a porous film is performed in the related art. As oil-water separation, a reverse osmosis method, an ultrafiltration method, a precise filtration method (for example, see Patent Document L2), and the like are also known.
However, because oil and water are separated by using a hole diameter of the separation membrane in the reverse osmosis method, the ultrafiltration method, and the precise filtration method, there is a problem in that a membrane permeation flux is small. Further, in the process of performing water treatment, a separation target substance such as oil, which is provided in raw water adheres to the separation membrane, and thus fouling (clogging) occurs. A problem in that it is necessary that physical washing such as back pressure washing and air scrubbing be periodically performed occurs due to the fouling. Thus, there is a situation in which improvement of difficulty in adhering oil (antifouling property) or ease of removing adhered oil (easy washing property) is desired for the separation membrane using a porous film in order to continuously use the separation membrane for a long term.